The mining industry is engaged in mining and recovering ores of metals and minerals from the ground, subsequently processing the ores and minerals to provide raw materials which are then further processed to obtain products of commercial use and value. The mined ores and minerals often contain the value metal compound or the commercially usable mineral in amounts less than 15 wt %, and therefore the ore or the mineral, needs to be subjected to various mineral separation and concentration processes. The mineral separation process usually comprises several process steps, eventually yielding a concentrate of the desired metal compound or mineral, and waste rock and tailings. The tailings may still contain low levels of the mineral or the value metal compound, however, the bulk of the tailings consists of gangue minerals, silicates, waste rock and other substances accompanying the ore or mineral in the ground. It is noted that when the value metal is in a form of a sulphidic compound, the tailings usually contain substantial amounts of iron sulphide which is often the host mineral of the value metal compound in the ground.
The tailings produced in the mineral separation process steps are often in a form of an aqueous slurry, such as for example, tailings resulting in flotation separation process steps, but they may also be dry. The particle size of tailings range from very fine to granular or sandy, often referred to as slimes and tailing sands, respectively. The tailings together with waste rock are usually regarded as waste products of mining processes. Tailings and similar mine waste products have often been deposited and left at the side of mining operations as tailing piles or dumps, or deposited in ponds or placed into cavities in the ground, such as mine shafts which may be subsequently flooded. Such remains of mining operations were often an eyesore, but most detrimentally, the oxidation products could lead to contamination of the regional surface and ground waters. Environmental regulations now require that a mining company takes responsibility for the visual appearance of the surroundings of the mine, and takes appropriate steps to ensure that the tailings and similar waste rock produced, and the effluent generated, do not contaminate the regional waters nor the environment. Furthermore, regulations also require that when a mine is decommissioned the mining area is left in such state that unpleasant, unsightly and harmful consequences of the mining operation, be that actual or potential, are controlled or eliminated. One of the most significant harmful effects of improper tailing management, is the possible chemical contamination of the regional waters. The regional waters may become contaminated as a result of escaped water soluble mining process reagents or oxidation products of substances in the mine waste products, such as remains of flotation separation and similar reagents in the tailings, but more particularly, as a result of atmospheric oxidation of the sulphides contained in the tailings, which can lead to acid generation and to the formation of sulphates and other heavy metal compounds which are subsequently solubilized and mobilized by rain. The water soluble compounds may interact with the surroundings of the tailing deposit in a manner that can cause contamination, unless properly intercepted and treated.
The subject of tailing management is discussed in several publications, and has become an important part of the initial mine design and mine operation, and the required closure planning conducted by responsible mining companies. A procedure sometimes practiced in handling tailings, is to feed a thickened slurry of the tailings at a point above the pile, allowing the tailings slurry to run down the sides of the pile. The tailing sands usually settle above the fines and the run-off water collects at the bottom. The run-off water may be treated subsequently to remove or neutralize harmful components. A method of treatment of mineral tailings is described in published Canadian patent application 2,090,141, which was filed by Peter Davies on Feb. 23, 1993. The treatment includes separating the tailings by physical separation process steps into a slime or fine fraction and a coarse fraction, then depositing alternating layers of these in the tailing deposit pile, thus allowing the effluent to run away more readily while the layers get compacted by their own weight. It is noted, however, that such tailing treatment methods are usually not recommended for sulphidic tailings and/or sulphide containing waste rock, which may yield undesirable oxidation products when exposed to atmospheric oxidation.
Another object of tailing management is to improve the appearance of tailing piles, by providing new growth surfaces for vegetation. Successful revegetation, however, is usually costly and the soil deposited on top of the tailings needs to be deep enough to isolate the roots of the plants from the underlying tailings, especially if the oxidation products generated in the tailings are detrimental to plant growth. The soil layer has to be able to sustain growth of the plants, however, a thick soil layer on top of a sulphide bearing tailing deposit may have little or no effect on the continued oxidation, nor on the solubilization of heavy metal salts and acid formation already present therein.
As discussed above, acid drainage caused by oxidation of sulphides in tailings and similar waste rock is considered a serious threat to the environment. Sulphidic minerals within an ore body will not oxidize as long as the ore body is not exposed to atmospheric oxygen in the presence of moisture. However, once the excavated sulphide bearing rocks and minerals are brought above ground and subjected to mineral separation processes, the sulphide in the tailing pile, waste rock and similar waste and by-products of mining processes, will be subject to oxidation, yielding sulphate and ferric ions which react with more pyrite in the tailings, thus promoting further in-situ oxidation and producing sulphuric acid and ferrous ion containing effluent or acidic drain water. The most significant oxidation reactions in sulphidic tailings may be summarized and represented by the following equations: ##EQU1## It can thus be seen that the predominating promoters of the oxidation of iron sulphides, in particular of pyrite, are oxygen and water. Similar equations may be written for the oxidation of pyrrhotite which is the other dominant sulphide mineral present in ore deposits. The rate and extent of these reactions are also influenced by other factors, including the presence of sulphur-oxidizing bacteria (Thiobaccilli ferrooxidans and Thiobaccilli thiooxidans) in the ground. The resulting acidic sulphate solution will react with other heavy metals present as sulphide or in other form, leading to heavy metal sulphate containing acidic drain water. Ferric ions generated will act as oxygen carriers in promoting further oxidation of the sulphidic minerals, thus further enhancing sulphuric acid formation, and leading to more acidic metal sulphate and similar salt containing solutions which can escape and contaminate the regional waters and lead to similar hazards detrimentally affecting the environment. The above discussion is intended merely as a brief summary of the more important mechanisms leading to acid mine drainage.
The above reactions taking place in sulphide bearing tailing deposits exposed to air and atmospheric moisture usually lead to the formation of three distinct zones: i) a top, highly acidic layer having pH less than 3, usually extending to a depth of 30-70 cm; ii) a hard layer, often referred to as `hard pan`, having thickness between 5 and 15 cm, containing gypsum, iron oxides, silicates and oxides or oxy-sulphates of other metals, which are usually formed by the interaction of calcium and silicate compounds present in the ground with sulphuric acid and salt containing drain water seeping from the top zone, and iii) a generally undisturbed zone of tailings which has not been oxidized hitherto, comprising unreacted sulphides, rock and other components of tailings. It has been suggested that the `hard pan` could protect the unaffected, deeper layers from further oxidation and acid drain formation. However, weathering, ice and snow in the winter and drought in the summer, and the inherent volume changes brought about by structure and composition changes in the course of the formation of the `hard pan`, are likely to result in erosion, breaks and cracks in the hard layer, which then can allow air and moisture to reach the previously unaffected zone, initiating further oxidation and acid drainage. It may be concluded that the `hard pan`, and by the same argument any hard covering layer, is unlikely to hinder reliably the oxidation of sulphidic tailing deposits and avoid the formation of acid ground water, in the long term. As mentioned above, reclaiming by revegetation sulphidic tailing deposits which have been previously exposed to oxidation, may fail, even if the top layer of the tailing deposit is covered by top soil and fertilizer. The roots of the growing plants and trees may penetrate below the top soil, into the oxidized tailing deposit, encountering water soluble salts and very acidic conditions, and may die after the first winter.
It has been observed that the oxidation of sulphides in mine tailings and similar mineral process waste products may be substantially diminished if the tailings and waste products are kept under water, such as in tailing ponds. However, if the water level in the tailing pond is not maintained, or the pond is allowed to dry out or drain away, thus allowing the oxidation of sulphidic compounds in the tailings to proceed, undesirable acid drainage is likely to be the result.
Another method proposed to prevent a harmful effluent generated seeping into ground water is to provide an impervious lining for a cavity containing the effluent generating waste material, this method, however, has serious drawbacks.
There are known processes for capping or providing a hard cover or sealing layer over municipal waste or industrial waste deposits, in particular for covering landfill sites and waste dumps. Composite layers utilized in such processes may be made of naturally occurring materials, such as gravelly moraine, loam, sand and similar substances, mixed with larger particles of gravel, sand and the like, in specific particle size distribution ranges. The layers made of such mixtures are frequently reinforced with a geotextile to provide a composite sealing layer. Such a process is described, for example, in U.S. Pat. Nos. 5,141,362 and 5,374,139, issued to J- U. Kugler on Aug. 25, 1992 and Dec. 20, 1994, respectively. The Kugler patents teach a self sealing cap for waste dumps, comprising mixtures of fine particles capable of flowing and sealing cracks that have formed. The cap supported on a geotextile mat, is overlain by a filter layer and a soil layer. Other processes for synthetic hard covers over municipal waste piles, admix building waste and binders such as cement kiln dust, bentonite, fly ash, portland cement and the like, which may also be mixed with cellulose or plastic fibres and other organic or processed carbon bearing materials. The covers are required to eliminate bad odour, or damage by birds and animals, or in some cases to contain effluent formation, but not to control oxidation. Furthermore, the effluent generated by municipal waste is not necessarily harmful to the environment and in any case, it can be treated to render it harmless. In contrast, acid drain water generated in the oxidation of sulphidic mine tailing deposits cannot be readily eliminated or treated. As was discussed above, methods to provide a hard cover is not considered to be applicable to deposits of sulphidic mine tailings and sulphidic waste rocks, as a protective or oxidation preventive measure.
A method for providing a cap for tailing ponds is described in U.S. Pat. No. 5,118,219, issued to D. D. Walker Jr. on Jun. 2, 1992. The cap is intended to prevent the drying out of such ponds and thereby eliminate dusting. The capping cover is made of lime, a sulphate component, a pozzolanic material and water, and allowed to harden.
It is known to introduce carbonaceous substances in direct contact with the sulphidic tailings to consume oxygen and thereby to prevent, or at least hinder, oxidation. In one form of utilizing a carbon containing material, in particular carbon in the form of dead vegetation and similar materials, the organic carbon containing material is mixed in with the sulphide bearing tailing particles, with the objective of acting in-situ as adsorbent for the acid produced as well as a reductant, thereby avoid oxidation. The treatment of sulphide bearing mine tailings by providing organic carbon layers in the tailing deposit directly below the water table and mixing particulate organic carbon source with the sulphide bearing tailing particles disposed above the water table, is described in Canadian patent 1,327,027 issued to Blowes et al. on Feb. 15, 1994. The position of the carbon layer and the amount of carbon mixed with the tailings are calculated based on the depth of the sulphidic tailings above the water table. The water table level may, however, shift with time and season, or all the carbon mixed in within the tailings may be consumed, thereby allowing the tailings to oxidize without hindrance.
There are several studies currently undergoing trials, involving various carbonaceous substances capable of consuming oxygen, directly overlaying sulphidic tailing deposits. Such carbon containing substances include wood shavings, compost, treated sewage sludge and municipal waste, dried and chopped vegetation, and substances of similar nature. However, whether a single capping layer containing predominantly carbon bearing substances is capable of preventing the oxidation of the underlying sulphide bearing deposit for a prolonged time period is not yet known. It may be assumed that if the organic carbon is not mixed in some way or anchored to a material which is able to retain water, the carbon containing material may dry out and be blown away. In other words, it seems that a predominantly carbonaceous material containing cover layer on top of the sulphidic tailings deposit is likely to lose its effectiveness in the long term.
There have been other known studies in which various natural particulate layers were provided over sulphidic tailing deposits, but those have not been shown to prevent the oxidation of the underlying sulphidic materials, or they have been found to require additional and costly effluent treatment.
It may be concluded from the above, that there is a need for an inexpensive method to eliminate or at least, substantially slow down the oxidation of sulphide bearing tailing and waste rock deposits.